Successful fabrication of biaxially textured superconducting wire based on the coated conductor technology requires optimization of the cost/performance of the HTS conductor. From a superconducting performance standpoint, a long, flexible, single crystal-like wire is required. From a cost and fabrication standpoint, an industrially scalable, low cost process is required. Both of these critical requirements are met by Rolling-assisted-biaxially-textured-substrates (RABiTS).
In order for cost/performance for a conductor based on this technology to be optimized, further work needs to be done in the area of buffer layer technology. It is now clear that while it is fairly straightforward to fabricate long lengths of biaxially textured metals or alloys, it is quite difficult to deposit high quality buffer layers using low cost processes. Requirements of buffer layers include—it should provide an effective chemical barrier for diffusion of deleterious elements from the metal to the superconductor, provide a good structural transition to the superconductor, have a high degree of crystallinity, excellent epitaxy with the biaxially textured metal template, have good mechanical properties, high electrical and thermal conductivity and should be able to be deposited at high rates.
Buffer layers play a key role in high-temperature superconductor (HTS) materials, particularly in second-generation rare-earth-barium-copper-oxide (REBCO), especially yttrium-barium-copper-oxide (YBCO) wire technology. The purpose of the buffer layers is to provide a continuous, smooth and chemically inert surface for the growth of the YBCO film, while transferring the biaxial texture from the substrate to the YBCO. Buffer layers are important for preventing metal diffusion from the substrate into the superconductor, and as oxygen diffusion barriers. Nickel can poison the YBCO layer, destroying the superconductive properties. To transfer the texture from the template to the superconductor while preventing the diffusion of nickel metal to the YBCO film, buffer layers are needed. These insulating layers also reduce both alternating current losses and the thermal expansion mismatch between the crystal lattices of the substrate and the superconductor. Multi-layer architectures have been developed that also provide mechanical stability and good adhesion to the substrate.
It is important that the highly aligned buffer materials are matched in both the lattice constant and thermal expansion to the biaxially textured substrate and the YBCO layer. The buffer layers should be smooth, continuous, crack-free and dense. Even though the buffer layers are much thinner than the YBCO layer, buffer deposition cost is a substantial part of the total conductor cost. Hence, there is a need for the development of economically feasible, efficient, scalable, high rate, high quality buffer layers and associated deposition processes.
When growing an epitaxial oxide film on a metal surface it is essential to avoid oxide formation before the nucleation of the layer is complete. For example YBCO is typically grown in an atmosphere containing 100 ppm or more of oxygen at 700-800° C. Under such conditions Ni and W will form various native oxides on a NiW surface. Most of such oxide layers do not offer the bi-axial cubic texture required for high critical currents in the HTS layer. However, the ability of certain oxide films to be grown in very low oxygen partial pressures, i.e. below the oxidation of Ni and even W can be utilized. A thin seed layer is deposited first, to subsequently allow the growth of the full buffer and finally the YBCO processing at higher oxygen levels.
Although it is possible to grow oxide buffers directly on textured metal surfaces under suitable reducing conditions, oxygen penetrates through the buffer layers such as yttrium oxide, cerium oxide, and lanthanum zirconium oxide (LZO) to the metal oxide interface during the YBCO processing. This is mainly because the oxygen diffusivity of the buffer layer materials at 800° C. is in the range of 10−9 to 10−10 cm2/sec. The diffusion is more rapid in structures that are more prone to the occurrence of defects. Grain boundaries, pinholes and particulates can also provide diffusion paths in these films. In most instances the oxidation of the metal substrate cannot be suppressed completely Thin homogeneous oxide layers are observed after final processing of the conductor without negative impact on the performance. Uncontrolled growth of substrate oxides, however, can result in multiple failures. Rapid and inhomogeneous oxide growth can penetrate the buffer layers and cause deterioration of the barrier properties of the buffer layer. For example, when excessive NiO is formed the full buffer layer stack can delaminate from the metal substrate due to volume expansion of, for example, Ni relative to NiO. The characteristics of the buffer layer can control the extent of oxide layer formation at the interface between the buffer and the substrate.
Buffer layers can be formed by multiple layers of materials which each serve a particular purpose. One example of a buffer layer stack of the prior art includes the use of YSZ and CeO2, typically in a configuration of CeO2/YSZ/CeO2. The purpose of the first buffer layer is to provide a good epitaxial oxide layer on the reactive, biaxially textured Ni substrate without the formation of undesirable NiO. CeO2 is particularly useful in this regard due to its ability to very readily form single orientation cube-on-cube epitaxy on cube textured Ni. Deposition of CeO2 using a range of deposition techniques can be done using a background of forming gas (4% H2-96% Ar) in the presence of small amounts of water vapor. Under such conditions the formation of NiO is thermodynamically unfavorable while the formation of CeO2 is thermodynamically favorable. The water vapor provides the necessary oxygen to form stoichiometric CeO2. Using CeO2 as a buffer layer one can readily obtain a single orientation, sharp cube texture. Ideally, the CeO2 layer would be grown thick such that it also provides a chemical diffusion barrier from Ni, followed by deposition of YBCO. However, when the CeO2 layer is grown greater than 0.2 μm in thickness, it readily forms micro-cracks. Hence a YSZ layer which does provide an excellent chemical barrier to diffusion of Ni and does not crack when grown thick is deposited on a thin initial template of CeO2. However, since there is a significant lattice mismatch between YSZ and YBCO, a second 45°-rotated orientation nucleates at times. In order to avoid the nucleation of this second orientation completely, a thin capping layer of CeO2 or another material is deposited epitaxially on the YSZ layer.
The fabrication of YBCO coated conductors requires an economic and robust process to produce the YBCO superconductor layer. The BaF2 ex situ (BF) method requires two distinct processing steps whereby first a non-superconducting precursor layer is deposited onto the substrate and this precursor layer is subsequently annealed in a furnace to form the superconducting YBCO. The two steps may facilitate economic scale-up for long length coated conductor fabrication. On a suitable substrate or buffer layer, using the BF method, the YBCO can be grown as an epitaxial layer with high critical current density (Jc).
Many buffer layer materials or substrates exhibit a deleterious chemical reactivity toward the precursor layer during the ex situ anneal to the extent that a high-Jc epitaxial YBCO film cannot be obtained. For example CeO2 exhibits a moderate reactivity toward the precursor layer (leading to the reaction product barium cerate, BaCeO3). The reaction of the CeO2 with YBCO always results in the formation of BaCeO3 at the buffer/YBCO interface. The use of CeO2 as a single buffer layer on metallic substrates is therefore restricted to metal (Ni) diffusion through the CeO2 into the YBCO. LaMnO3 (LMO) appears to exhibit a reduced reactivity toward the BF precursor during the ex situ anneal, making it easier to form epitaxial, high-Jc YBCO layers. It has also been demonstrated that LMO provides a barrier against Ni diffusion and can be used as a single buffer layer. A single LMO layer was expected to replace both the YSZ and CeO2 layers and possibly even the first seed layers.
LaMnO3 perovskite (LMO) has been extensively studied in the past years. Recent work has demonstrated that LaMnO3 and (La,Sr)MnO3 perovskites are good buffer layers for growth of superconductors such as YBCO. This work suggests that LMO could be used as a single buffer layer on Ni and Ni—W substrates for growth of YBCO by either pulsed laser ablation or the ex-situ BF technique. It was also demonstrated that LMO could be grown epitaxially on MgO single crystal. This is important since it enables growth of such buffer layers on ion-beam assisted deposition (IBAD) or (ISD) produced, biaxially textured MgO on untextured metal substrates.